CBSE Class 8 Science Chapter 02 The Invisible Living World Beyond Our Naked Eye MCQs Set 02

Practice MCQs for Class 8 Science Chapter 02 The Invisible Living World Beyond Our Naked Eye

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Question: In Activity 2.2, when preparing to observe onion peel cells under a microscope, what was the primary purpose of using glycerin after the safranin staining step?
A. To increase the visibility of the nucleus
B. To prevent the cells from drying out and enhance clarity during observation
C. To remove the excess stain from the preparation
D. To harden the cell wall for better structural support
Show Answer & Explanation

Answer: (B) To prevent the cells from drying out and enhance clarity during observation

Explanation:
The chapter explicitly states that glycerin was added 'to prevent drying of the cells and improve clarity for better visualisation of cells.' This is a protective and optical measure taken before placing the coverslip.

Question: Robert Hooke's 1665 observations of cork tissue led him to coin the term 'cell' because the empty spaces he saw reminded him of what?
A. The compartments of a storage box
B. A honeycomb structure
C. The chambers of an animal heart
D. Water droplets arranged in a pattern
Show Answer & Explanation

Answer: (B) A honeycomb structure

Explanation:
The text states: 'These compartments reminded him of a honeycomb. He drew what he saw and called each small space a cell.' The honeycomb analogy directly inspired the scientific terminology.

Question: According to the chapter, what is the key limitation that prevents most microorganisms from being visible to the human eye without optical instruments?
A. They do not produce pigments
B. They are too small in size
C. They are transparent like glass
D. They move too rapidly to be observed
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Answer: (B) They are too small in size

Explanation:
The introductory passage establishes: 'The human eye can only see objects that are above a certain size.' The chapter repeatedly emphasises that microorganisms, by definition, 'can not be seen with the naked eye' because of their microscopic dimensions.

Question: Which of the following correctly pairs a component found in plant cells with its primary function as described in the chapter?
A. Vacuole—stores waste products only and cannot be used for storing useful substances
B. Chloroplast—contains chlorophyll and enables the cell to make food through photosynthesis
C. Cell wall—controls the entry and exit of materials into and out of the cell
D. Mitochondria—provides the green colour that allows plants to absorb sunlight
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Answer: (B) Chloroplast—contains chlorophyll and enables the cell to make food through photosynthesis

Explanation:
The 'A step further' section identifies chloroplasts as structures 'containing chlorophyll, which makes them green and helps in photosynthesis.' The other options misstate functions: the vacuole stores important substances and nutrients, the cell membrane (not wall) controls material exchange, and mitochondria are cellular powerhouses unrelated to colour.

Question: In Activity 2.4 and Activity 2.5, students observed tiny organisms in both pond water and soil suspension. What does this observation suggest about the distribution of microorganisms in nature?
A. Microorganisms exist only in water environments
B. Microorganisms are found in diverse environments including water and soil
C. Microorganisms are more abundant in soil than in water
D. Microorganisms are randomly distributed with no predictable pattern
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Answer: (B) Microorganisms are found in diverse environments including water and soil

Explanation:
Both activities revealed microorganisms in different habitats—pond water and soil suspension—and the chapter states microbes 'are found all around us—in water, soil, air, and even inside our body.' This demonstrates their widespread presence across varied environments.

Question: Based on Activity 2.7, when fruit and vegetable peels were mixed with soil and left for 2–3 weeks, they transformed into manure. Which process is primarily responsible for this transformation?
A. Physical breakdown of organic matter by weather
B. Decomposition carried out by bacteria and fungi present in the soil
C. Chemical reaction between peels and minerals in the soil
D. Evaporation of water content from the peels
Show Answer & Explanation

Answer: (B) Decomposition carried out by bacteria and fungi present in the soil

Explanation:
• Soil contains various microorganisms (fungi and bacteria)
• These microbes act on plant waste and break it down
• The chapter explains: 'microorganisms breakdown the complex substances of fallen leaves into simpler substances that are rich in nutrients' in a process called decomposition.

Question: In Activity 2.3, human cheek cells were observed to have a polygonal shape, whereas onion peel cells from Activity 2.2 appeared nearly rectangular. What does this structural difference primarily indicate?
A. Cheek cells lack a cell wall while onion cells possess one
B. Cheek cells are more advanced in their evolutionary development
C. Polygonal and rectangular shapes serve identical functions
D. Human cells always lack a nucleus while plant cells always contain one
Show Answer & Explanation

Answer: (A) Cheek cells lack a cell wall while onion cells possess one

Explanation:
The chapter notes that onion cells have a cell wall (giving them firm, rectangular appearance when packed together), while human cheek cells have only a cell membrane and cytoplasm without a cell wall. The cell wall in plant cells provides rigidity and causes compact, firm arrangement, whereas animal cells without this rigid outer layer form more irregular polygonal shapes when arranged in tissue.

Question: According to the 'Ever heard of' section on Antonie van Leeuwenhoek, what distinction earned him the title 'Father of Microbiology'?
A. He invented the first microscope ever made
B. He was the first person to clearly see and describe tiny living things like bacteria and blood cells
C. He discovered that cells could be classified into plant and animal types
D. He developed the theory that all living things are made of cells
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Answer: (B) He was the first person to clearly see and describe tiny living things like bacteria and blood cells

Explanation:
The text states: 'He was the first person to clearly see and describe tiny living things like bacteria and blood cells. Because of this, he is known as the Father of Microbiology.' This historical recognition reflects his pioneering work in observing and identifying microscopic organisms.

Question: In Activity 2.9, milk placed in a warm environment (bowl A) curdled while milk in a cool environment (bowl B) did not curdle, even though curd starter was added to both. What does this outcome demonstrate?
A. Lactobacillus bacteria cannot survive in cold temperatures
B. Temperature is an important environmental factor affecting the growth and metabolic activity of Lactobacillus
C. Curd formation is independent of environmental conditions
D. Cold milk prevents bacteria from entering the liquid
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Answer: (B) Temperature is an important environmental factor affecting the growth and metabolic activity of Lactobacillus

Explanation:
The chapter explains: 'These bacteria grow well in warm conditions. That is why curd is formed in bowl A but not in bowl B.' The experiment isolates temperature as a variable while keeping other factors constant, showing that Lactobacillus requires warmth to metabolise lactose and produce lactic acid for curd formation.

Question: The chapter describes certain bacteria called Rhizobium living in root nodules of legume plants. What mutually beneficial relationship is established between Rhizobium and legume plants according to the text?
A. Rhizobium obtains nutrients from the plant while the plant receives fixed nitrogen that improves its growth
B. Rhizobium produces chlorophyll while the plant provides shelter in nodules
C. Rhizobium decomposes dead plant tissue while the plant absorbs the nutrients
D. Rhizobium manufactures water while the plant supplies carbon dioxide
Show Answer & Explanation

Answer: (A) Rhizobium obtains nutrients from the plant while the plant receives fixed nitrogen that improves its growth

Explanation:
The chapter states: 'These bacteria trap nitrogen from the air and make it useful for the plants. This helps plants grow better without chemical fertilisers.' Meanwhile, the bacteria themselves reside within the plant's nodules, receiving carbohydrates and other compounds necessary for their survival, creating a symbiotic exchange.

Question: What distinction does the chapter make between yeast fermentation in Activity 2.8 and Lactobacillus fermentation in Activity 2.9 in terms of the products they generate?
A. Yeast produces acid while Lactobacillus produces alcohol
B. Yeast produces carbon dioxide and alcohol while Lactobacillus produces lactic acid
C. Both produce identical products but at different temperatures
D. Yeast produces bubbles while Lactobacillus produces taste changes only
Show Answer & Explanation

Answer: (B) Yeast produces carbon dioxide and alcohol while Lactobacillus produces lactic acid

Explanation:
The chapter explicitly contrasts the two processes: yeast 'produces a small amount of alcohol during this process' along with carbon dioxide that 'forms bubbles,' whereas Lactobacillus bacteria 'produce lactic acid, which makes curd sour' instead of producing alcohol. These different metabolic products lead to distinct results—fluffy bread versus sour curd.

Question: The chapter's discussion of microalgae in section 2.4.3 emphasises their importance by stating they 'produce more than half of the Earth's oxygen supply.' What does this claim suggest about the relationship between microscopic life and planetary conditions?
A. Microalgae are too small to have any significant environmental impact
B. Tiny organisms play a disproportionately large role in maintaining conditions necessary for life on Earth
C. All oxygen on Earth is produced exclusively by microalgae
D. Oxygen production requires organisms that are visible to the naked eye
Show Answer & Explanation

Answer: (B) Tiny organisms play a disproportionately large role in maintaining conditions necessary for life on Earth

Explanation:
By highlighting that organisms invisible to our eyes are responsible for producing over half of atmospheric oxygen, the chapter illustrates that microscopic life has profound large-scale ecological significance. This challenges the assumption that small organisms have negligible importance to planetary systems.

Question: According to the chapter, why is crop rotation with legumes (as described in the context of Rhizobium bacteria) beneficial for soil health without requiring chemical fertilisers?
A. Legumes absorb toxic chemicals from the soil
B. Nitrogen-fixing bacteria in legume nodules naturally increase soil nitrogen content
C. Legumes produce their own nitrogen through photosynthesis
D. Rotation prevents soil erosion but does not affect nutrient levels
Show Answer & Explanation

Answer: (B) Nitrogen-fixing bacteria in legume nodules naturally increase soil nitrogen content

Explanation:
The text explains: 'This naturally increases the nitrogen in the soil and keeps it healthy for the next crop.' Rhizobium bacteria trapped in nodules 'trap nitrogen from the air and make it useful for the plants,' enriching the soil naturally without synthetic inputs. This is why farmers practice legume rotation—to maintain soil fertility.

Question: What does the placement of an 'Ever heard of' section on Spirulina and its farming in the chapter suggest about the chapter's broader purpose?
A. To discourage students from pursuing microbiology careers
B. To present microorganisms solely as threats to human health
C. To illustrate how scientific knowledge of microbes can be applied to address practical human needs like food security and livelihoods
D. To explain why microalgae are unsuitable for human consumption
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Answer: (C) To illustrate how scientific knowledge of microbes can be applied to address practical human needs like food security and livelihoods

Explanation:
The Spirulina section transitions from discussing microalgal diversity and conservation to emphasizing 'farming of Spirulina is becoming a feasible livelihood opportunity' and 'ensures food security and livelihood development.' This demonstrates how understanding microorganisms moves beyond abstract biology into real-world applications that benefit people, aligning with the chapter's theme of connecting invisible life to human welfare.

Question: In the chapter's explanation of why pickles and murabbas do not become infected by microorganisms, what is the protective mechanism described?
A. The sealed container prevents all air from entering
B. High concentrations of salt or sugar inhibit microorganism growth
C. The acidity of the pickling liquid kills all bacteria present
D. Cold storage temperatures prevent any microbial activity
Show Answer & Explanation

Answer: (B) High concentrations of salt or sugar inhibit microorganism growth

Explanation:
The text states: 'This is because you add many spices with salt or sugar to it which act as preservatives. High concentration of salt or sugar do not allow these organisms to grow on them.' The osmotic effect of high solute concentrations creates an environment hostile to microorganism reproduction.

Question: What does the chapter's comparison of cell structure to a brick wall in Activity 2.2 primarily illustrate?
A. That cells are non-living structures similar to bricks
B. That the cell is the smallest independent unit of life, just as the brick is the smallest structural unit of a wall
C. That plant cells are rectangular while animal cells are round
D. That microscopes magnify objects to the same degree as human eyes magnify distance
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Answer: (B) That the cell is the smallest independent unit of life, just as the brick is the smallest structural unit of a wall

Explanation:
The analogy establishes a hierarchy of structural organisation: 'Cell (Fig. 2.7a), is the basic unit of life, just like a brick is the basic unit of wall (Fig. 2.3d).' This metaphor communicates that cells are irreducible building blocks from which all larger biological structures are constructed, parallel to how bricks are fundamental to wall construction.

Question: When Robert Hooke observed a thin slice of cork tissue under his early microscope, what specific visual characteristic reminded him to call the tiny structures he saw 'cells'?
A. They were arranged in rows like soldiers in formation
B. They resembled the small compartments found in a honeycomb
C. They appeared to be hollow and connected like tubes
D. They contained liquid that moved around like water in individual chambers
Show Answer & Explanation

Answer: (B) They resembled the small compartments found in a honeycomb

Explanation:
Hooke named these structures 'cells' because the small, empty spaces he observed in the cork reminded him of a honeycomb's compartments. This comparison captures why the term 'cell' was adopted into scientific language to describe the basic unit of life.

Question: When observing cheek cells in Activity 2.3 versus onion peel cells in Activity 2.2, the cheek cells appeared polygonal while the onion cells appeared nearly rectangular. What does this structural difference reveal?
A. Cheek cells are younger and have not yet developed rigid cell walls
B. The shape of cells is related to their specific location and function in the body
C. Polygonal cells are more advanced and contain more complex internal structures
D. Animal cells always appear polygonal regardless of their type or location
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Answer: (B) The shape of cells is related to their specific location and function in the body

Explanation:
The shape difference reflects how cells are adapted to their roles: onion peel cells are tightly packed and protective, so they maintain a rectangular, brick-like arrangement, while cheek cells form a flexible lining and thus appear polygonal to allow better fitting together without gaps. Cell shape is a direct consequence of cellular function.

Question: In Activity 2.8, why was it necessary to add both sugar and warm water to the flour before adding yeast, rather than adding yeast to dry flour?
A. Sugar and warm water provide the optimal conditions for yeast to respire and release carbon dioxide
B. They chemically activate the yeast cells so they can divide more rapidly
C. They dissolve the yeast powder completely, making it easier to distribute throughout the dough
D. They prevent the yeast from producing alcohol, keeping the bread from becoming intoxicating
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Answer: (A) Sugar and warm water provide the optimal conditions for yeast to respire and release carbon dioxide

Explanation:
• Yeast requires warm conditions to grow well and metabolise efficiently
• Sugar provides food for yeast cells to break down during respiration
• During respiration, yeast releases carbon dioxide gas, which creates the bubbles that make dough rise
• Without these optimal conditions, yeast remains inactive and fermentation does not occur

Question: Based on Activity 2.9, the observation that curd formed in bowl A (warm milk with curd starter) but not in bowl B (cold milk with curd starter) most directly demonstrates which principle?
A. Lactobacillus bacteria can only survive in warm environments and die in cold conditions
B. Temperature influences the metabolic rate and growth of microorganisms
C. Curd starter cultures are ineffective unless they are heated before addition
D. Cold temperatures denature the proteins in milk, preventing curd formation
Show Answer & Explanation

Answer: (B) Temperature influences the metabolic rate and growth of microorganisms

Explanation:
This activity shows that while Lactobacillus bacteria were present in both bowls, they grew and fermented the milk to form curd only in warm conditions. The bacteria did not die in the cold—they simply became metabolically inactive, illustrating that temperature directly controls microbial growth rates and activity levels.

Question: In Activity 2.7, when fruit and vegetable peels were mixed with soil and left for 2–3 weeks, they gradually transformed into dark-coloured manure rich in nutrients. Which sequence of steps best explains how this transformation occurs?
A. Fungi and bacteria break down complex plant materials into simple nutrient-rich compounds through decomposition
B. Soil minerals chemically react with plant peels under heat and pressure to create manure
C. Microorganisms consume the plant waste and excrete solid manure as a byproduct
D. Plant cells decompose naturally over time as their cell membranes rupture from moisture
Show Answer & Explanation

Answer: (A) Fungi and bacteria break down complex plant materials into simple nutrient-rich compounds through decomposition

Explanation:
Fungi and bacteria in soil act as decomposers, breaking down the complex molecules in dead plant material through respiration and metabolic activity. This process, called decomposition, converts the waste into simpler substances rich in nutrients that can be used by other plants. Manure formation depends on microbial action at appropriate temperature and moisture levels.

Question: According to the chapter's discussion of microalgae in section 2.4.3, why are these microscopic organisms considered critical for maintaining Earth's environmental balance, despite being invisible to the naked eye?
A. They convert carbon dioxide into oxygen faster than all terrestrial plants combined
B. They produce more than half of Earth's oxygen supply while also serving as a food source for aquatic life
C. They are the only organisms capable of absorbing excess carbon dioxide from the atmosphere
D. They prevent climate change by reflecting sunlight away from the Earth's surface
Show Answer & Explanation

Answer: (B) They produce more than half of Earth's oxygen supply while also serving as a food source for aquatic life

Explanation:
Microalgae play two essential roles: they produce over half the planet's oxygen supply through photosynthesis, and they serve as a foundational food source for aquatic ecosystems. This dual importance makes them essential for both atmospheric composition and food webs, despite their microscopic size.

Question: In the 'Ever heard of' section explaining why pickles and murabbas resist microbial infection, what is the key protective principle involved?
A. High salt or sugar concentrations inhibit the growth of microorganisms that would otherwise spoil the food
B. Spices used in these foods contain natural antibiotics that kill all bacteria and fungi
C. The pickling process heats the food to temperatures that sterilise it completely
D. Microorganisms cannot survive in acidic environments created during fermentation
Show Answer & Explanation

Answer: (A) High salt or sugar concentrations inhibit the growth of microorganisms that would otherwise spoil the food

Explanation:
The chapter explains that high concentrations of salt or sugar create an environment where microorganisms cannot grow, even if they are present. This is a preservation method based on osmotic pressure—these high solute concentrations do not allow microbial cells to function, without needing to kill them outright.

Question: The chapter contrasts the role of yeast in Activity 2.8 with the role of Lactobacillus in Activity 2.9. In terms of fermentation products, which statement accurately captures this difference?
A. Yeast produces lactic acid while Lactobacillus produces carbon dioxide gas
B. Yeast produces both alcohol and carbon dioxide, while Lactobacillus produces primarily lactic acid
C. Both microorganisms produce the same products but at different rates depending on temperature
D. Lactobacillus produces alcohol while yeast produces carbon dioxide for bread rising
Show Answer & Explanation

Answer: (B) Yeast produces both alcohol and carbon dioxide, while Lactobacillus produces primarily lactic acid

Explanation:
Yeast respires and ferments sugars to produce both carbon dioxide (which creates bubbles and makes dough fluffy) and alcohol (which gives bread its distinctive smell). Lactobacillus, by contrast, produces lactic acid as its main fermentation product, making milk sour and transforming it into curd. These different metabolic outcomes determine their different culinary applications.

Question: When observing onion peel cells in Activity 2.2, glycerin was added to the stained preparation on the glass slide before placing the coverslip. Based on the chapter's explanation, what would most likely happen to the cell visibility if glycerin had been omitted from this step?
A. The cells would appear more clearly defined because there would be no liquid to obscure them
B. The cells would dry out quickly, causing them to shrivel and become difficult to observe under the microscope
C. The safranin stain would wash away completely when the coverslip was placed
D. The nucleus would become permanently invisible because glycerin is required to stain it
Show Answer & Explanation

Answer: (B) The cells would dry out quickly, causing them to shrivel and become difficult to observe under the microscope

Explanation:
The chapter explicitly states that glycerin prevents drying of the cells and improves clarity for better visualisation. Without glycerin, cells lose moisture rapidly under the microscope's light, causing them to shrivel, collapse, and become too distorted to observe clearly.

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